The present invention relates to an electricity measurement apparatus and a method of electricity measurement, in particular for current measurement, power measurement and Watthour metering.
When current flows through a conductor, a rotational magnetic field is set up. The magnitude of this magnetic field is proportional to the density of current flowing: EQU B.alpha.I/A (Equation 1)
where I is the total current, and A is the cross sectional area of the conductor. This value I/A is the current density J.
It is known to use sensors, such as Hall effect sensors, to measure current flow though a conductor by detecting the magnetic field. For example, using a Hall effect sensor, the sensor output signal has a voltage, VH, which is proportional to the magnetic field strength B at the sensor, and a bias current IB flowing through the sensor as follows: EQU VH=S.times.B.times.IB (Equation 2).
The proportionality coefficient S is the sensitivity of the sensor. By keeping constant the current through the Hall sensor, a linear relationship is achieved between the detected magnetic field and output voltage.
If such a sensor is placed near a conductor, the two equations (1) and (2) above can be combined allowing current flow to be determined: EQU VH.alpha.J (Equation 3).
Hall sensors produce Direct Current (D.C.) offset errors, which are independent of the direction (positive or negative) with which the bias current IB is applied to the sensor. Conventionally, as shown in FIG. 3, the bias current IB is selected to be proportional to the electricity mains supply voltage. The current IB and detected magnetic field B are both Alternating Current (A.C.) signals which vary according to a sine relation. Accordingly, the output voltage VH follows a sine squared relation where D.C. offsets are components of the resultant VH signal.
Other D.C. offsets can also occur due to the electronic circuits associated with the subsequent processing operations.
The effects of D.C. offsets are minimised conventionally by using large Hall sensors, in particular so called "external" sensors not integrated with other processing circuitry on an integrated circuit such as a silicon chip. The conventional principle applied is that the larger the Hall sensor is, the larger is the Hall voltage which it provides relative to its D.C. offset error.